I've got 3 battle decks I can customize, but when you start a battle you we always seem to be using deck 1. How can you select a different deck? Will this unlock at a specific arena? If you cannot, what is the use of the other decks?
You can see numbers 1 2 3 and the active deck is differentiated with a different color. Just touch any number whose deck you want to use.
Then, just battle on. That is it. A touch on one of those 3 numbers activates that deck. Then you're ready to go battle with that deck.